1. Field of the Invention
This invention relates to the conversion of carbonaceous materials into synthesis gas. This invention relates to a method and system for conversion of carbonaceous materials into synthesis gas. More particularly, this invention relates to low temperature gasification of carbonaceous materials to produce synthesis gas. More particularly yet, this invention relates to low temperature gasification of carbonaceous materials using a low temperature Plasma Assisted Reforming (PAR) process for the co-production of hydrogen-enriched fuel gas, chemicals and electricity as an alternative to high temperature thermal gasification. This invention further relates to a method and system for conversion of carbonaceous materials to synthesis gas using non-thermal plasma reactors.
2. Description of Related Art
Methods and systems for gasification of carbonaceous materials to produce synthesis gas are well known. See, for example, U.S. Pat. Nos. 4,057,402, 4,369,045, and 5,092,984, all of which are directed to coal gasification; U.S. Pat. No. 3,891,403 directed to gasification of oil shale; and U.S. Pat. Nos. 4,699,632 and 4,592,762 directed to gasification of biomass. Indeed, gasification of coal is one of the oldest methods for producing hydrogen. In conventional gasification processes, the coal to be converted is heated up to about 900° C., at which temperature it turns into a gaseous form, after which it is mixed with steam and fed over, or otherwise brought into contact with, a catalyst.
A plasma is a collection of ions, electrons, charge-neutral gas molecules, and other species in varying degrees of excitation resulting from the separation of gas component molecules of a gas to which a specific amount of energy has been added. Depending on the amount of energy added, the plasma can be characterized as thermal or non-thermal. In a thermal plasma, enough energy is introduced so that the plasma constituents are in thermal equilibrium—the ions and electrons are, on average, at the same temperature. Exemplary of a thermal plasma generator is an electric arc, of which a lightning bolt bridging the gap between a storm cloud and the earth is a familiar manifestation. The temperature of thermal plasma components is about 1-2 electron-volts (1 eV corresponds to about 11,600° K.). A non-thermal plasma is one in which the mean electron energy, or temperature, is considerably higher than that of the bulk-gas molecules. Because energy is added to the electrons instead of the ions and background gas molecules, the electrons can attain energies in the range of about 1-10 eV, while the background gas remains at ambient temperature. This non-thermal condition can be created at atmospheric, sub-atmospheric and super-atmospheric pressures. Exemplary of a non-thermal plasma generator is a dielectric barrier discharge.
The use of electrical discharge, i.e. plasma, to initiate chemical reactions of interest is also known and has been in use for a long time. For example, U.S. Pat. No. 4,352,044 teaches a plasma generator in which a mixture of a gaseous oxidizing agent, such as steam and oxygen, and a pulverized solid fuel is supplied directly into a zone in which an electric discharge produced by a DC and AC source is sustained for the purpose of gasifying the pulverized solid fuel. The plasma generator comprises a discharge chamber provided with means for introducing a plasma-forming medium and associated with a cathode assembly and an anode assembly, the latter of which includes at least two plasmatrons, each having a hole for an inlet for the plasma-forming medium and being provided with an end electrode and an auxiliary hollow electrode. These electrodes are connected to an arc discharge initiating system. The exit openings of the auxiliary electrodes communicate with the discharge chamber and are evenly distributed along the perimeter of its cross section. The cathode assembly also comprises at least two plasmatrons, each having a hole for an inlet for the plasma-forming medium and being provided with an end electrode and an auxiliary hollow electrode, each of which is connected to an arc discharge initiating system.
U.S. Pat. No. 6,923,890 B2 teaches a method for activating chemical reactions using a non-thermal capillary discharge plasma unit or a non-thermal slot discharge plasma unit.
A dielectric barrier discharge (DBD) is a gas discharge (a non-thermal plasma) between two electrodes separated by one or more dielectric layers and a gas-filled gap. When a high voltage is applied to the electrodes, the electric field in the gap ionizes the gas. The ions and electrons produced by this electric discharge are attracted towards the electrodes of opposite polarity and form a charge layer on the dielectric surface. These charges cancel the charge on the electrodes so that the electric field in the gap falls to zero and the discharge stops. U.S. Pat. No. 6,326,407 B1 and U.S. Pat. No. 6,375,832 B1 teach a method of transforming a normally gaseous composition of methane into a material comprising a major portion of hydrocarbons containing at least two carbon atoms using a dielectric barrier discharge; U.S. Pat. No. 6,896,854 B2 teaches a reactor for reactive co-conversion of heavy hydrocarbons and hydrocarbon gases to lighter hydrocarbon materials which includes a dielectric barrier discharge plasma cell; and U.S. Pat. No. 6,146,599 teaches a dielectric barrier discharge system having first and second non-thermal plasma reactors coupled together in series, which system is indicated to be used to decompose hazardous compounds in a liquid or a gas, such as in power plant flue gases.
In catalytic gasification of coal, simultaneous use of an external heat supply and catalytic promotion of the reaction is employed to reduce the gasification temperature. Catalytic gasification of coal has thermal characteristics similar to the catalytic steam reforming of natural gas. Coal contains significant amounts of inorganic matter (ash), primarily Si, Al, Fe, Ca, Mg, Na, K and Ti oxides. Some of those components (K, Na, Fe, and Ca) have been reported to be catalysts in solid fuel conversion reactions. To realize this catalytic effect, the coal surface must be continuously activated by thermal or chemical treatment. Studies have shown that a catalyst could decrease coal gasification reaction temperatures to 700° C. The idea of using plasma for coal surface activation at the process temperature typical of catalytic gasification originates from these studies.
Low pressure cold plasma studies have shown increases in coal reactivity, but at near room temperature. Gasification of solid fuels at near-room temperature was studied in 2.45 GHz low-pressure (600-3000 Pa) microwave discharges in batch and continuous flow reactors at temperatures up to 100° C. The main reaction products were H2, CO, CO2, CH4 and C2H6, with H2 and CO being the most abundant. The addition of water vapor increased the syngas output.
Several groups have also studied coal conversion in a glow discharge plasma. Plasma treatment of bituminous coal (60 Hz, 26 kV, 2.6 mA) yielded H2 and CO, with trace amounts of CH4. Electronic Spin Resonance (ESR) data showed a 2,5-fold increase in spin concentration (1018 spins/g) indicating a radical reaction mechanism. Gasification of anthracite by CO2 in direct current (100 mA) glow discharge plasma at 5,340 Pa pressure was also studied. These studies show that plasma species activate the coal surface and initiate surface reactions of coal conversion at near room temperature. Plasma discharges in H2O and CO2 convert coal into syngas. In a hydrogen-rich environment, the plasma discharges produce methane and higher hydrocarbons.
In atmospheric pressure cold plasma studies, increases in reactivity were shown, but at temperature levels below those of the invention described herein below. Two groups have evaluated gasification in non-thermal plasmas at atmospheric pressure and process temperatures up to 350° C., wherein the coal was gasified using a corona discharge in hydrogen. The temperature was varied between 100° C. and 350° C. The results showed a similar yield as for thermal decomposition, except that no tar was produced. Another study of gasification of tars in pulsed corona discharges in H2O, CO2 and H2 at temperatures up to 200° C. has also been reported. The results show the feasibility of tar gasification at 200° C. and that H2O was the most effective agent for tar decomposition, followed by CO2 and H2.